Before you purchase a light fixture for your aquarium there are a few items you should know.  By understanding these items it will help you make a more educated and appropriate choice for lighting your aquarium.

1. Understanding the different types of lights/bulbs available:
   The most popular and common types of fixtures available to light your aquarium include Normal Output fluorescent (NO), Very High Output fluorescent (VHO), Power Compact fluorescent (PC), T5 High Output (T5HO) fluorescent and Metal Halide (MH) light fixtures.  The brightest of these fixtures are the Metal Halide fixtures and the least bright are the NO florescent fixtures.
   
2. Matching the right light to your aquarium:
   For a fish only aquarium you can use any type of lighting you wish to.  The most popular type of lights for a fish only system is the NO fluorescent fixtures.  They generally will be the least expensive and will provide enough light for viewing your fish.  If you are going to be keeping live plants or live corals in the tank you will need a lighting system that will provide plenty of light for the plants and symbiotic algae found in corals to photosynthesize.  This can be accomplished using multiple NO output bulbs over the tank or high output lights such as a VHO, PC or T5 fluorescent fixture or a MH system.
   
3. Spectrum of the bulbs:
   Bulb will be available in different spectrums or colors.  The spectrum ratings are listed by a Kelvin (K) temperature scale.  The lower end of the K range bulbs you will normally see are 5500K-6700K bulbs.  These bulbs will generally be a greenish (5500K) to a yellow color (6700K) in color and are a good spectrum for growing live plants.  The next most common color seen will be the 10000K bulbs.  These are generally a crisp white color and while not as good for your plants they will still provide beneficial light without the yellow look.  As you go higher in K rating (12000K-20000K) the light will start taking on a more bluish hue.  Generally the higher K bulbs are used in saltwater setups.  Most of your NO bulbs will simply be labeled “color enhancing” or “plant grow” bulbs for an easier selection.  For a fish only system, the color enhancing bulbs are generally your best choice.
   
4. How much light is needed?
   For a fish only system there is no real rule of thumb to go by.  The light is mainly there for you to be able to view your fish.  As long as the bulb(s) extend the full length of your aquarium you should be fine.  For planted aquariums the wattage of light needed will depend on the type of fixture used and the depth of the tank.  For lower output lights like NO fluorescents a good starting point would be about 2-5 watts per gallon of water.  Higher output lights like VHO, PC and T5HO should be good with around 1-3 watts per gallon of water.  For metal halide fixtures you will need one bulb for every two feet of tank length and the wattage will depend on the depth of the aquarium.  A general guide line would be for tanks under 22” deep go with 175 watt bulbs, tanks 22-28” deep use 250 watt bulbs and for tanks over 28” tall use 400 watt bulbs.
   
5. How long should your lights be kept on?
   For a fish only system you can keep your lights on for as long as you wish with the average time being around 4-8 hours per day.  If you are having issues with severe algae growth in the aquarium you can lower the amount of time the light is on to help inhibit the algae growth.  For live plants the lights should be run from 10-14 hours per day to allow the plants plenty of time to photosynthesize.
   
6. Use timers for a more stable environment in your aquarium.
   Fish are very much creatures of habit.  They enjoy things staying fairly stable including the lighting schedule.  By putting your lights on timers to come on and off the same time every day the fish will generally be less stressed and overall healthier.
   
7. When considering the price of a lighting fixture, you should also look at the replacement costs of the bulbs.
   You should not wait for your bulbs to burn out before replacing them.  Bulbs will slowly dim over time and many times this is not noticeable to the human eye.  The lights over a planted aquarium should be replaced every 8-12 months and for a fish only system the bulbs should be changed every 1 to 2 years.
   
8. You will need to consider how to mount the lights over your aquarium.
   There are two different types of fixtures available, fully assembled and retrofit kits.  A fully assembled fixture is basically ready to go right out of the box.  Most will come with mounting legs or hanging kits.  Mounting legs will allow you to set the fixture right on the rim of your aquarium while the hanging kit will allow you to suspend the light over your tank.  If you have a canopy over your aquarium usually a retrofit kit is your best option.  It will mount right inside the canopy with a few screws.
   
9. So you want to light a reef tank?
   What is the ideal lighting for a reef tank?  While this is a highly debated topic from the type of light (metal halide vs. VHO vs. PC vs. T5) to the spectrum of the bulbs you must figure out what works best for your tank and the type of corals you plan on keeping.  If you are mainly going to be keeping soft corals (like leathers, mushrooms and xenia) you probably don’t have to invest in a metal halide system for your corals.  If you are keeping SPS corals (such as Acropora and/or Montipora), clams and anemones you will want to give them as much light as possible.  Research the lighting requirements of the photosynthetic organisms you want to keep in your tank and find a lighting system that suits their needs.  If you are not sure what type of corals you want to keep it is probably better to go with a higher wattage fixture to be on the safe side.
   
10. What is your lighting system going to cost you (on your electric bill).
    Since everyone has different electric rates in different parts of the country (and world) it is hard to give an exact estimate of how much running a given fixture will cost.  To help get a rough estimate you can take the wattage of the fixture (add up the wattage of the different bulbs) and using your electric rate you can get an approximation.  There is a factor to consider also and that is the type of ballasts used.  Electronic ballasts such as IceCap ballasts can help reduce the electrical consumption of a lighting system.  For example their fluorescent ballasts will use about 30% less electricity compared to conventional fluorescent ballast.  Also don’t forget that your lighting system will not be running 24/7 like other items on your reef tank.  Running the lights for 8-12 hours per day in most cases will be perfect.  Just for an example if your electric rates are $0.11 KWH and you are running a system with 500 total watts for 10 hours per day it will cost you about $15 per month to run that system.